Conventionally, a protection element which is mounted on a secondary battery device or the like is employed as the one that has a function of preventing an overvoltage as well as an overcurrent. This protection element is formed so that: a heating element and a soluble conductor made of a low-melting metal member are laminated on a board; the soluble conductor is blown out due to an overcurrent; and in case that an overvoltage is generated as well, power is supplied to the heating element in the protection element and then the soluble conductor is blown out due to a heat of the heating element. Blowout of the soluble conductor takes place due to goodness of wettability relative to a surface of a connected electrode at the time of blowout of the soluble conductor that is a low-melting metal. The low-melting metal that has been blown out is attracted onto an electrode, and as a result, the soluble conductor is broken and then a current is shut off.
On the other hand, with downsizing of an electronic device, such as a portable device, in recent years, there has been a need for downsizing or thinning a protection element of this type; and there has been a further demand for operational stability and fastness. As a means therefor, there is provided the one in which a soluble conductor of a low-melting metal member is disposed on an insulation board; the thus disposed soluble conductor is sealed with an insulation cover; and a flux is coated onto the soluble conductor. This flux is adapted to prevent oxidization of a surface of the soluble conductor, and is provided so that the soluble conductor blows out speedily and stably at the time of heating the soluble conductor.
Such a protection element has a structure shown in FIG. 13. In this protection element, a pair of electrodes 2 is provided on a baseboard 1, and a pair of electrodes, although not shown, is provided at an opposite edge part which is orthogonal to the electrodes 2 as well. A heating element 5 made of a resistor is provided between electrodes, although not shown, and a conductor layer 7 which is connected to one of the pair of electrodes, although not shown, via an insulation layer 6, is provided. At this protection element, a soluble conductor 3 made of a low-melting metal foil is provided between the pair of electrodes 2 that is formed on both ends of the baseboard 1. A center part of the soluble conductor 3 is provided on the conductor layer 7. Further, an insulation cover 4 is provided in face-to-face opposite to the soluble conductor 3 that is provided on the baseboard 1. The insulation cover 4 which is mounted on the baseboard 1 is put with a predetermined space 8 being formed relative to the soluble conductor 3. A flux 9 is coated onto the soluble conductor 3, and the flux 9 is housed in the space 8 which is provided in the insulation cover 4.
In addition, as disclosed in Patent Document 1, as a protection element for shortening a circuit shutoff time due to coagulation at the time of blowout of a low-melting metal member and then reducing a difference in operation time, there is provided the one in which a low-melting metal member having two stripes or a low-melting metal member forming a slit in an intra-electrode direction is provided between a pair of electrodes supplying a current to the low-melting metal member. This protection element is capable of segmenting the low-melting metal member between the electrodes in an independent state, increasing the number of blowout start points in low-melting metal member, and then, reducing and stabilizing an operation time.